Well your right on more than one count! The Johnson/Evinrude 10/15hp are the same except for carburetor, slight port/timing changes and the majority of the boost to 15hp coming from the tuned exhaust system on the last 10% of the throttle twist. Johnson built--up to a few years ago--a mini hydro ‘racing’ version of the 15hp motor. Aside from the radical lower unit and prop it seemed to be mostly a highly tuned exhaust system that gave this motor it’s 'zing'.

Sorry, don’t have any info. on converting the 10hp to the 15hp though…